1. Field of the Invention
This invention relates to light emitting diodes (LED), particularly to the packaging of LED for low thermal resistance.
2. Brief Description of Related Art
A traditional LED package is shown in FIG. 1. A LED chip 10 is mounted in a recess of a lead frame 11. The recess reflects the light emitted from the LED chip 10. The LED 10 is wire bonded with a gold wire or an aluminum wire 14 to another lead frame 11 to serve as a second terminal for the LED 10. The LED 10 is coated with transparent glue, leaving only the leads exposed for external connection. Such a LED package is widely used for traffic signal lights, signal lights in a car, and indicators in an electronic production line.
Another prior art is shown in FIG. 2. A LED chip 10 is mounted on a printed circuit board 13, and is wire bonded with gold or aluminum wire 14 to surface contact leads of the printed circuit board. The structure is covered with transparent glue 12. Phosphorescent powder may be added to the glue to produce white light.
The forgoing structures have the shortcoming that the chip is coupled with bonded wires. Such bonding wires all have some degree of pliancy and strength, and widely used for semiconductor packaging. Due to the high price of gold, the cost is substantial in production. In consideration of the area and light transmission, the bonding wire is usually limited to 0.8 to 1.5 mils in diameter. The stress, which such wires can withstand, is limited, especially for the currently popular solderless bonding. The stress produced by the solderless bonding causes higher temperature at the bond, which, in turn, may cause breakage. It is therefore desirable to eliminate the use of bonding wire.
These two structures all have the drawback that the thermal resistance is excessive. Since the chip is a source of heat. When heating sinking is inadequate, the LED chip temperature rises, shortens the life of the LED, reduces the brightness of the light, and even causes the light source to be ineffective. Therefore, the heat removal or heat sinking of the LED is an important consideration for the LED package.
In general, the heat sinking property of a LED package is determined by its thermal resistance. Since the heat source of the package is solely from the LED chip, we often use the path from the P-N junction of the LED to the package lead to define the thermal resistance RθJ-P. It is the thermal resistance from the junction to the lead pin. Mathematically,RθJ-P=(TJ−TP)/Qwhere TJ is the light emitting diode junction temperature,                TP is the lead line temperature,        Q is the heat flux.Since the light emitting diode chip is the sole source of heat generation, and only a negligible amount of this energy is radiated as electromagnetic waves, the bulk of the energy is transformed into heat. Thus the thermal-resistance formula can be rewritten as:RθJ-P=(TJ−TP)/(If−If)where If is the operating current of the LED and Vf is the operating voltage.        
Since the pin temperature is determined by the ambient temperature of the system and is not affected by the heat sinking property of the LED, one can see from the foregoing formula that the junction temperature increases with increasing thermal resistance.
From the standpoint of conduction heat transfer, the thermal resistance can be expressed as:Rθ=L/(K×A),where L is the length of the heat conduction path, K is the thermal conductivity coefficient, and A is cross-sectional area of the heat conduction path.
Thus, we can see that the longer the heat conduction path, the larger the cross-sectional area and lower the thermal conductivity coefficient, the higher is the thermal resistance. Therefore, for low thermal resistance design, it is important to shorten the heat conduction path, to increase its cross-sectional area and to select a material with high thermal conductivity coefficient.
The foregoing two prior art LED packages essentially dissipate the heat through the lead frame or the printed circuit board. The printed circuit board shown in prior art FIG. 2 is made of plastic material, which has a very low thermal conductivity coefficient, and is incapable of dissipating heat. The printed copper wire on the circuit board has only a thickness of tens and hundreds um with small cross-sectional area. Hence, the thermal resistance is very high, ranging 500-1000° C./Watt. When the energy is high, the LED can easily be overheated. Fir the prior art described in FIG. 1, the heat path through the support frame of copper or iron. Although the heat conductivity coefficient is good, the cross-sectional area is still very small, ranging 150-250° C./W. Thus the load current can only be approximately 30 mA
To remedy this problem, other approaches have been suggested. FIG. 3 shows a design with enlarged pins to reduce the thermal resistance. However, the conduction path is long and can only achieve a thermal resistance of 50-75° C./W.
Another invention disclosed in U.S. Pat. No. 6,274,924 and shown in FIG. 4. A set of lead frame 11 (not referenced in the figure) is imbedded in an insulating mold 15. Inside the mold 15 is a conduit which has an enlarged flange 16 serving a heat sink. The LED chip 10 is mounted on a submount 17 which is attached to the heat sink, serving as the lead for the bottom electrode. The top electrode of the LED is wire bonded to the another lead frame. The heat sink reduces the heat conduction path and enlarges the cross-sectional area to reduce the thermal resistance to 10-15° C./W. However, from a production stand point, the additional heat sink increases the production processing step and the height of the package. Another problem is that when large LED chips are used, the packaging area need to be increased. An increase in packaging area increases the stress caused by different expansion coefficients, and the danger of breaking the bonding wire also increases.